(define WINDOW-WIDTH 150)
(define WINDOW-HEIGHT 180)
(define MAX-DIAMETER 40)

; A world is a number.
; Its display is a blue disk of that size.
; show-world : world -> image
(define (show-world diameter)
  (circle diameter "solid" "blue"))
"Examples of show-world:"
(show-world 1) "should be a blue dot"
(show-world 20) "should be" (circle 20 "solid" "blue")

; The next world is 1 larger than this one, unless we've reached MAX-DIAMETER,
; in which case stop the timer with the current world.
; next-world : world -> world
(define (next-world diameter)
  (cond [(< diameter MAX-DIAMETER) (+ 1 diameter)]
        [else (end-of-time)]))
"Examples of next-world:"
(next-world 23) "should be" 24
(next-world MAX-DIAMETER) "should be something special indicating the end of time"

(big-bang WINDOW-WIDTH WINDOW-HEIGHT 1/2 1)
(on-update-event show-world)
(on-tick-event next-world)